DE1913171A1 - Process for the production of hot rolled steel for further processing by pressing - Google Patents

Description

H. MK. IF!

Nippon Kokan Kabushiki Kaisha 1913171

Tokyo «Japan

Process for the production of hot rolled steel for further processing by pressing

The invention relates to a method for producing

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hot-rolled steels with good pressability, like them required for compression molding processes. According to the invention, this method is characterized in that the material with the formation of a crystal structure and without the possibility of recrystallization over the entire length of steel strips or strips in one treated in a predetermined range of finish rolling and coiling temperatures and then by simultaneous annealing or annealing the whole strip is recrystallized.

A good press deformability around steel strips via a hot process To lend, high finish rolling and coiling temperatures have hitherto been used. Especially in the case of thin steel strips or sheets with a thickness of less than 1.6 mm, however, it is due to a temperature gradient difficult to maintain the high temperatures mentioned during the rolling process. Of course, this tendency appears even more clearly as the strip thickness decreases even further. When the final rolling temperature decreases, it increases with it the reel temperature also decreases accordingly.

Even if it were possible to increase the reel temperature,

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the condition of the on a roller table deteriorates running belt. For this reason there is also no point in deliberately increasing the coiling temperature. Consequently it may become impossible to use hot-rolled strips or steel sheets produced under these conditions Presses far too process. Since the mechanical Properties of such sheets tend to be unstable, cold-rolled steel sheets have heretofore been used for press forming.

However, it is well known that there has long been a need for hot rolled steel sheets having better press formability.

The object of the invention is therefore primarily to create a novel and improved method for Manufacture of hot-rolled steels or steel sheets with good press formability instead of cold-rolled Steel can be used.

This object is erfindungegemäß in a method of producing hot-rolled steel with good press formability achieved in that a steel strip or band at a lower final rolling temperature of 400 - 55O ⁰ C is hot rolled to a - 73O ⁰ C and a lower coiling temperature of 300 To obtain crystal structure without recrystallization over the entire length of the steel strip or band, and that the steel strip or band is then annealed or tempered at a temperature above the recrystallization point of steel.

The invention is explained in more detail below using the example of drawings. Show it

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Pig. 1 is a graph showing the relationship between the final rolling temperature of the product and its elongation;

Pig. 2 is a photomicrograph of the crystal structure produced in a conventional hot rolling process;

Pig. 3 is a photomicrograph of a rolling process, the intermediate stage of the present invention Process representing the expanded structure and

Pig. 4 is a photomicrograph of one produced by known annealing or tempering processes, the final stage of the process according to the invention representing the recrystallization structure.

According to the invention, the material passes through the last rolling stand with a lower temperature of 4OQ - 73O ⁰ C and then at a still lower temperature of 300 - 55O ⁰ C coiled.

As mentioned, the reason that such is chosen from 73O ⁰ C as the upper limit of the finish rolling temperature, in that at a last stitch overlying a temperature, even if only local, recrystallization occurs. If, on the other hand, the temperature of the last pass is below 400 ^° C., it is often impossible to carry out the rolling process with the usual facilities of a hot strip mill, which also results in increasing surface defects in the roll and the running strip or strip.

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The coiling temperature range of 300 - 55O ⁰ C is from the estate, chosen because at temperatures ⁰ C often recrystallization occurs from above 55O during cooling to below 300 ⁰ C, thereby coiling the strip or tape is impossible! while at the same time numerous surface defects form on the belt surface.

It is a known fact that about a quarter or a third of a normally rolled strip is from the beginning of the strip complete recrystallization occurs even in the case of rolling to a thickness of less than 0.8 mm. if a coil with such a structure mentioned on the usual Wise annealed or tempered, the uniformity of the mechanical properties of the tape is about its Total length canceled. For this reason it is ■ · " required, the occurrence of a completely recrystallized th structure, even locally, to be prevented by both the final rolling as well as the coiling temperature in sufficient Level to be lowered.

The task of annealing or tempering consists in the implementation a perfect recrystallization of the hot-rolled steel strip or » -bands and in a softening of the belt. The annealing or tempering temperature range must therefore be above the recrystallization point of the strip. In this case, the said Annealing or tempering of the hot-rolled strips or strips if necessary also after the usual pickling.

The method according to the invention described above is explained in more detail below on the basis of test results explained. The two types of steel examined were manufactured and owned using conventional wind-freshening processes following chemical composition:.

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Steel grade

0.04
0.08

0.26 0.34 0.009 0.013

0.016 0.020

example 1

Comparative example (steel by conventional method

rolled)

Thickness:

Steel grade:

Final rolling temperature:

approximately

Coil temperature: approximately annealing or tempering temperature:

0.8 mm steel A 740 ⁰ C (starting area of

hot rolled strip)

680 ⁰ C (middle area of the band) 520 ⁰ C

700 ⁰ C (batch annealing or tempering)

The mechanical properties obtained under the above conditions are listed in Table 1 below.

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Table 1

Beginning area of the band middle area of the band

when- hot after annealing- rolled or tempered after annealing rolled or tempering

Stretch limit
(kg / mm ^) 36.9 29.9 34.4 15.2 tensile strenght
(kg / mm ² ) 37.6 33.4 41.2 27.8 Stretching (#) 28.8 33.7 15.7 43.7 Erichsen-
Value (mm) 9.3 9.6 8.4 10.6 Cone bowl
Value (mm) 39.8 39.5 41.0 38 »1 lankgord
Value (r) 0.80 0.80 0.60 0.61

From Table 1 it can be readily seen that the mechanical properties show a considerable difference between the beginning and the middle of the belt. This result is due to the inequality of Endwal temperature ζ in the two Bereiohen due, 74o C. welohe for the beginning of the tape and is 680 ⁰ C for the central region. In particular, it can be determined with certainty that the crystal structure has completely recrystallized after rolling under the conditions given above. This structure is in Pig. 2 illustrates.

Example 2
(Method according to the invention)

Thickness »Steel grade:

Final rolling temperature reel temperature:

Annealing or tempering temperature j

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0.8 mm steel A 67000

about 500 ^U C 700 ⁰ C

The following table shows the mechanical properties of the material rolled and annealed or annealed under these conditions. tempered bands on.

- Table 2 after glow
or starting Midrange after
respectively. of the tape Start of tape area 18.7 warm
rolled the glow
Tempering warm
rolled 28.8 55.2 18.4 Stretch limit
(kg / mm ² ) 55.7 45.4 • 42.0 29.4 tensile strenght
(kg / mm * ² ) 41.5 10.2 15.9 40.8 Stretching (#) 15.5 58.7 8.1 10.1 Erichsen-
Value (mm) 8.1 0.59 41.0 58.7 Cone bowl
Value (mm) 41.1 0.59 -0.59 Lankford
Value (F) o, 6o

It is readily apparent from Table 2 that the uniformity the mechanical properties in all areas of the belt are very satisfactory. Fig. 5 illustrates the crystal structure after rolling, i.e. in the state before complete recrystallization. One after Good recrystallization structure obtained after annealing is shown in FIG.

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191317t

Example 3
(Method according to the invention)

1.2 mm steel. B 68O ° C about 500 ⁰ C

Thickness:

Steel grade:

Final whale temperature:

Reel temperature:

Annealing or tempering temperature:

Table 3: Mechanical properties under the above conditions. ■; ■ -. ■

• After rolling. After glowing or
Tempering Stretch limit
(kg / mm ² ) 32.0 ■ - - »
.24.5 tensile strenght
(kg / mm) 40.9 32.3 Stretching ($>) 22.2 -49.8 Erichsen-
Value (mm) 9.8 12.6 Cone bowl
Value (mm) 49.6 47.0 Lankford
Value (r) 0.61 0.62

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ORlGiNAL JMSPECTßD

191317]

As is clear from Table 3, the mechanical Properties improved to such an extent that the hot-rolled strip made in accordance with the present invention does not turn out to be any Can be compared more to a conventionally hot-rolled strip. fig. Fig. 1 shows the manner of improving the stretching based on the above examples by annealing or tempering after hot rolling.

According to the invention, hot-rolled steel sheets with good press formability can therefore be produced at lower production costs produce and obviously use for certain purposes, for which so far usually only cold-rolled Steel sheets were used.

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Claims (3)

Patent claims 1. A method for producing hot-rolled steel with good press formability, characterized in that a steel strip or -hand at a Endwalztemperatiir of 400 - is hot rolled 55O ⁰ C, a crystal structure without recrystallization about - 73O ⁰ C and a coiling temperature of 300! the total length of the steel strip or band away, and that the steel strip or band is then annealed or tempered at a temperature above the recrystallization point of steel. 2. The method according to claim 1, characterized in that the annealing temperature is about 700-770 ⁰ G ". 3. The method according to claim 2, characterized in that the final rolling temperature about 680 ° C and the coiling temperature is about 520 ⁰ C. 909843/1130